--- title: "Mixing devices for coagulant" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:coagulant-mixing-devices" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/coagulant-mixing-devices site: BCT Engineering --- # Mixing devices for coagulant Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2070 Asar, 8 marks - Video: [Water Treatment | Chemical Feed Equipment and Flash Mixing](https://www.youtube.com/watch?v=YJnN9sJP62U) — American Water College - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/coagulant-mixing-devices - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question With neat sketches, describe the various types of mixing devices used in mixing the coagulant with water. ## Short answer Mixing devices for coagulant include hydraulic types (baffled channels, weir/hydraulic‑jump, in‑line injection), mechanical rapid‑mix tanks with impellers, and compressed‑air diffusers. Rapid mixing ensures uniform dispersion in 30–120 s before flocculation. ## Answer Mixing (rapid mixing) must spread the coagulant quickly and uniformly through the whole of the water within 30 seconds to 2 minutes, so that the charge is neutralised before large flocs begin to form. The usual devices are as follows. #### 1. Hydraulic mixing (no power) **(a) Mixing channel with baffles (baffled channel)**: the water flows round the vertical baffles or over and under horizontal baffles; the turbulence at each turn mixes the chemical. Velocity 0.3 to 0.45 m/s. ```text -->| |-->| |-->| |--> ----+ +--+ +--+ +----> (over-and-under baffles) ``` **(b) Hydraulic jump (weir) mixing**: the coagulant is dropped at a Parshall flume or a weir where a hydraulic jump forms. Simple and cheap, but the head is lost. ```text coagulant | ~~~~~~~~~~v~~\ water weir \ --> crest \___/\/\/~~~~~~~ --> to flocculator jump (turbulence) ``` **(c) Pipe (in-line) mixing / venturi**: the coagulant is injected into a pipe at a throat or a bend. #### 2. Mechanical mixing **(a) Rapid mix tank with a paddle or propeller (turbine)**: a square or circular tank with a motor-driven impeller; detention 30 to 60 s, velocity gradient $G=300$ to $1000\ \text{s}^{-1}$. ```text [Motor] | ______|______ | | | <- water in | --+-- | | paddle | -> to flocculator |_____________| ``` **(b) Flash mixer with deflecting baffle plates**: a high-speed impeller, with baffle plates on the tank walls that stop swirling and increase turbulence. #### 3. Compressed-air mixing Air bubbles through diffusers in the bottom of the tank cause turbulence. The mixer is followed by the **flocculator** (slow mixing), using baffled channels or paddles moving at 0.1 to 0.5 m/s for 20 to 40 min, where the flocs grow. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What must rapid mixing achieve? A. Grow large, strong flocs over 20 to 40 minutes B. Settle the flocs at 0.3 to 0.45 m/s C. Spread the coagulant uniformly through all the water within about 30 s to 2 min, before large flocs form D. Remove dissolved gases from the water Answer: C. Rapid mixing neutralises charge quickly and evenly; floc growth is the job of the later flocculator. (Key point: Purpose of rapid mixing) 2. Which set contains only hydraulic (no power) mixing devices? A. Paddle tank, propeller tank and diffused air B. Baffled channel, paddle tank and air diffusers C. Weir, turbine mixer and flash mixer driven by a motor D. Baffled channel, hydraulic jump at a weir or Parshall flume, and in-line pipe/venturi mixing Answer: D. Hydraulic mixers use the flow's own turbulence; paddles, turbines and compressed air need power. (Key point: Hydraulic mixing devices) 3. What are the typical design values for a mechanical rapid-mix tank? A. Detention 30 to 60 s and velocity gradient G of 300 to 1000 per second B. Detention 20 to 40 min and G of 300 to 1000 per second C. Detention 30 to 60 s and velocity 0.1 to 0.5 m/s D. Detention 30 to 60 min and G of 10 to 30 per second Answer: A. Rapid mix is short and intense: 30 to 60 s with G of 300 to 1000 per s. (Key point: Mechanical rapid mix tank) 4. How does the flocculator that follows the mixer differ? A. It is a fast mix of 30 s where the coagulant is first spread B. It is a slow mix at 0.1 to 0.5 m/s for 20 to 40 min, where flocs grow C. It uses compressed air to bubble the coagulant in D. It removes the settled flocs by backwashing Answer: B. Flocculation is gentle and long so flocs can grow without breaking. 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